The present invention relates generally to electrical resonators. More particularly, this invention relates to filters that utilize a plurality of such resonators for radio applications where relatively small size is important.
There are many applications where it is necessary to provide a relatively small, low-loss filter for radio frequency signals. One such application is in modern communications systems, where it is desirable to provide a radio transceiver which packs higher performance and greater efficiency into a package having smaller size and lighter weight. One of the major limiting elements in the design of such radio transceivers is the use of one or more bandpass filters at the incoming and outgoing radio frequencies. Such bandpass filters are often realized using so-called Transverse Electromagnetic (TEM) mode filters. (As will be seen in a moment, the term "TEM mode" is merely a convenient approximation.)
Several arrangements for providing such filters are known. One such arrangement utilizes air as the dielectric for each of one or more resonators in the filter. Size constitutes a major disadvantage with such a filter. This disadvantage is further aggravated since such a filter must also be made with relatively heavy walls in order to adequately support the relatively large overall size of the structure.
A second known arrangement utilizes a solid ceramic dielectric having a relatively large dielectric constant. This second arrangement offers a size reduction for the filter by a factor corresponding to the square root of the relative dielectric constant of the ceramic material with respect to that of air. That is, by a factor roughly equal to the square root of the relative dielectric constant of the material.
In each of the above arrangements, the actual mode that exists is really a "quasi-TEM" mode. This mode is not a pure, true TEM mode because the skin-effect causes a longitudinal electric field to exist within the conductors that, in turn, causes a longitudinal electric field to exist in the dielectric. By the well-known boundary condition theorem, tangential electric fields across interfaces (i.e., between a conductor and the dielectric) must be continuous. Therefore, the lowest order transmission line mode that exists is the Transverse Magnetic (TM) mode, not the TEM mode.
In most conventionally constructed filters, this deviation from a true TEM mode is so small as to be ignorable, hence the term "quasi-TEM". This approximation also holds true for the two known arrangements given above, where they are constructed to have a dielectric separation thickness (whether air or solid ceramic) of at least 5 skin depths so that the majority of the magnetic fields lines are constrained to be within the dielectric, and minimally within the conductors. This constraint ensures that the small amount of magnetic field existing within the skin-layer of the electrical conductors is kept small as to be ignorable. In so doing, the relative wave velocity, also known as velocity factor, is a function dependent only upon the permittivity and the permeability of the dielectric medium.
Thus the prospect of further size reductions, having a factor comparable to the above, hinges on the availability of new materials being developed or discovered that have even higher dielectric constants, or permittivity constants greater than currently available.
Accordingly, there exists a need for another method of effecting further reductions in the size and weight of filters intended for use in radio applications, including mobile, and particularly portable, applications.